MISFETs used as switching elements in an inverter circuit or a power supply circuit are largely categorized into a planar type MISFET and a super junction type MISFET. The planar type MISFET includes, e.g., a drain layer, an n-type base layer arranged on the drain layer, a p-type base layer formed in a surface layer portion thereof, an n+-type drain layer and an n+-type source layer formed in a surface layer portion of the p-type base layer in a spaced-apart relationship with each other. A gate electrode is arranged to face a surface of the p-type base layer existing between the n+-type source and the drain layer through a gate insulation film.
On the other hand, as disclosed in International Publication No. WO 2010/024433, the super junction type MISFET includes, in addition to the configurations of the planar type MISFET, a p-type column layer extending from the p-type base layer toward the drain layer. This structure helps reduce on-resistance and increase switching speed. One of the problems posed in the super junction type MISFET resides in that a reverse recovery time trr of a parasitic diode is long. The parasitic diode is formed by a p-n junction between the p-type base layer and the p-type column layer and the n-type base layer. In the super junction structure provided with the p-type column layer, a lot of carriers are stored in the p-type column layer. Therefore, when the parasitic diode is turned off, a large reverse recovery current attributable to the carriers flows for a relatively long period of time.
In a power module configured by serially connecting a high-side switching element and a low-side switching element, the reverse recovery current induces a through-current. Thus, the reverse recovery current flowing for a long period of time leads to an increased loss of energy. For example, if a large reverse recovery current flows through the parasitic diode of the low-side switching element while the high-side switching element is kept turned on, a large through-current is induced and, therefore, a loss of energy is increased.
International Publication No. WO 2010/024433 provides a solution to this problem. More specifically, in the invention of International Publication No. WO 2010/024433, a trap level (recombination level) is locally formed within an n-type base layer between a p-type column layer and an n+-type drain layer by performing baryon irradiation, thereby shortening the reverse recovery time trr. The trap level is formed locally and is not formed in the p-n junction portion between the p-type base layer and the p-type column layer and the n-type base layer. Accordingly, a leak current is not increased during an off-time period.
Another problem inherent in the super junction type MISFET is the hard recovery of the parasitic diode. By the term “hard recovery,” it is meant that a change of the reverse recovery current (di/dt) is fast. In the super junction type MISFET, depletion layers are expanded from not only the p-type base layer but also the p-type column layer when the parasitic diode is turned off. In particular, the depletion layer expanded from the p-type column layer rapidly joins the depletion layer expanded from another p-type column layer adjoining thereto and rapidly reaches the drain layer lying just below the depletion layer. For that reason, the current is changed sharply and the reverse recovery current is interrupted at a high speed. As a result, the reverse recovery current shows a steeply-varying large-amplitude oscillatory waveform (ringing). These reverse recovery characteristics (hard recovery characteristics) generate a lot of noise and may possibly cause an erroneous operation of a controller for supplying a control signal to the MISFET. In an inverter circuit, among others, for driving an inductive load such as an electric motor, the parasitic diode is turned on and off. Thus, the hard recovery characteristics at the time of turning off the parasitic diode become a problem.
The invention of International Publication No. 2010/024433 succeeds in shortening the reverse recovery time trr but fails to provide a solution to the hard recovery problem. Accordingly, problems remain unsolved when the invention is applied to a power module (a power module for driving an inductive load among others).